(1) Field of the Invention
The present invention relates to a p-type semiconductor and particularly to a p-type semiconductor with low resistance.
(2) Description of the Related Art
In recent years, a nitride semiconductor based on gallium nitride has been commercially utilized for a full color display, a blue (long wave ranged from ultraviolet to yellow) light-emitting diode used for a signal light and the like, a write/read apparatus for optical recording media and blue laser used for a laser microscope and the like. Further, the nitride semiconductor is expected to be applied to media which enables high-density record and white light source which uses no mercury. Additionally, since an oxide semiconductor based on zinc oxide has a band gap of 3.37 eV at room temperature, resistance to reducing gas, stability at a high temperature, and excellent transparence, it is expected to be applied to a transparent conductive film and like besides the above-mentioned media which enables high-density record, although it is not yet commercially utilized.
There are some problems to realize and spread these technologies. The biggest problem is to realize a p-type semiconductor with low resistance. To lower the resistance of a p-type semiconductor, it is necessary to increase hole carriers in a valence band which contribute to conduct electricity. To increase the hole carriers in the valence band, it is general practice to dope an acceptor dopant. However, an acceptor level of a nitride semiconductor used to realize these technologies is formed at a deep level in a forbidden band since electronegativity of nitrogen included in nitride semiconductor is large. Therefore, it is impossible to obtain a p-type nitride semiconductor with high concentration of hole carriers even though an acceptor dopant is doped. For example, gallium nitride doped with magnesium as an acceptor dopant has activation energy of about 200 meV. Even gallium nitride doped with beryllium, which has the least activation energy, as an acceptor dopant, has activation energy of about 85 meV. The activation energy is extremely high compared with thermal energy of about 26 meV at room temperature and therefore it is impossible to obtain a p-type gallium nitride semiconductor with high concentration of hole carriers. Additionally, an acceptor level of an oxide semiconductor is similarly formed at a deep level in a forbidden band since electronegativity of oxygen included in an oxide semiconductor is large. Therefore, it is impossible to obtain a p-type oxide semiconductor with high concentration of hole carriers even though an acceptor dopant is doped. For example, zinc oxide doped with nitrogen as an acceptor dopant has activation energy of about 200 meV. The activation energy is extremely high compared with thermal energy of about 26 meV at room temperature and therefore it is impossible to obtain a p-type zinc oxide semiconductor with high concentration of hole carriers.
As the background art to solve these problems, there are “Manufacturing method of a p-type semiconductor crystal and a light-emitting device” (refer to Japanese Laid-Open Patent application No. 14-289918), “Transparent conductive film of zinc oxide” (refer to Japanese Laid-Open Patent application No. 14-50229), “p-type Group III nitride semiconductor and manufacturing method and semiconductor device thereof” (refer to Japanese Laid-Open Patent application No. 14-353144), and “Growth method of p-type ZnO oxide semiconductor layer and manufacturing method of semiconductor light-emitting element using the method” (refer to Japanese Laid-Open Patent No. 14-93821). These methods, which relate to methods for lowering resistance of a p-type semiconductor, make an acceptor level shallow and lower resistance of a p-type semiconductor by doping an acceptor dopant and a donor dopant at the same time, and forming a composite which is made of an acceptor dopant and a donor dopant in host material.
However, there is a problem that a p-type semiconductor with satisfactorily low resistance cannot be obtained by conventional methods for lowering resistance of a p-type semiconductor. For example, according to a recent experiment report of Professor Yoshida et al. of Osaka University, gallium nitride doped with magnesium as an acceptor dopant and with oxygen as a donor dopant at a rate of 2 to 1 has activation energy of about 50 meV. The activation energy is extremely high compared with thermal energy of about 26 meV at room temperature and therefore it is impossible to obtain a p-type gallium nitride semiconductor with satisfactorily low resistance.